Annual Report, 9/12/08 Summary paragraph: Our section directs clinical trials of recombinant toxins at the NIH and in the lab studies the interaction of these toxins with the patient's malignant and normal tissues. Therapy of patients with chemotherapy resistant hematologic malignancies has been remarkably effective with Fv-toxins recombinant immunotoxins) targeting CD25 or CD22 in hairy cell leukemia and efficacy is also observed in other hematologic malignancies. The anti-CD22 recombinant immunotoxin BL22 produces complete remissions (CRs) in a high percentage of patients with hairy cell leukemia who have a low chance for CR with conventional therapies. Patient samples are studied in the lab to better predict and understand recombinant toxin efficacy, to better understand the role of cytotoxic T-cells observed in hairy cell leukemia patients, to study novel and established tumor markers in patients to better quantitate overall tumor burden, and to develop new assays of minimal residual disease. Our goal is to develop bacterial toxins which are engineered to kill cancer cells in patients who cannot be cured by standard therapy. The binding domain of the bacterial toxin, usually Pseudomonas exotoxin (PE), is replaced with a ligand, either a growth factor or an Fv fragment of a monoclonal antibody (MAb), which binds to a tumor associated antigen. Thus the recombinant toxin binds to and is internalized by malignant cells and causes cell death after the catalytic domain of the toxin enters the cytosol of the host cell. BL22 is a recombinant immunotoxin containing an anti-CD22 Fv fused to truncated PE. We had previously shown in the lab that BL22 was capable of inducing complete regressions of human CD22+ tumor xenografts in mice at plasma concentrations tolerated in monkeys and in killing malignant cells freshly obtained from patients with CD22+ B-cell leukemias. We completed accrual of 46 patients in a phase I trial of BL22 in patients with B-cell tumors, 31 of whom had hairy cell leukemia (HCL). Of these 31 HCL patients, 19 (61%) achieved CR and 6 (19%) had partial responses (PR). A phase II study was designed to retreat only those HCL patients not achieving hematologic remission to cycle 1. We observed 25% CRs and 25% PRs after 1 cycle, and 47% CRs and 25% PRs overall. Patients with moderate splenomegaly (up to 200 mm), have a significantly higher response rate (95%) compared to those with prior splenectomy (44%) or those with larger spleens (20%). BL22 has an excellent risk-benefit ratio in HCL since no patients died and all toxicity resolved. BL22 is also being tested in patients with pediatric acute lymphoblastic leukemia. HA22, a variant of BL22 with higher affinity for CD22, is also being developed. We have compared this molecule with BL22 against cells ex vivo from patients with CLL and have found significant improvements in efficacy. HA22 has begun clinical testing in 3 separate multicenter phase I trials, for HCL, chronic lymphocytic leukemia (CLL) and non-Hodgkin's lymphoma (NHL). Major responses included 6 (38%) CRs and 6 (38%) PRs out of 16 evaluable HCL patients. Responses were observed at all dose levels including 3 of 3 PRs at the lowest dose level, and 1-2 CRs at each of the next dose levels. The maximum tolerated dose has not yet been determined since no patient has had severe toxicity. We found that soluble CD22 could be detected in the serum and its level correlates closely with overall disease burden. Another phenomenon studied is expansions of cytotoxic T-lymphocytes (CTLs), associated with limited repertoire of polyclonal T-cells, related to purine analog treatment. BL22 treatment had no adverse effect on T-cells. Monitoring CTLs in patients with HCL after purine analogs may be very useful to gauge the T-cell repertoire and could be used to help decide whether relapsed patients should receive a repeated course of purine analog or instead try biologic T-cell-sparing therapy. It is known that purine analogs cannot eliminate HCL cells. By cloning the cDNA for surface IgG displayed on the hairy cells, we have developed a novel sequence-specific PCR test for HCL. While flow cytometry, currently the most sensitive test of minimal residual disease (MRD) in HCL, is able to detect 1 HCL cell in 10,000 normal, clone-specific quantitative PCR is able to detect 1 HCL cell in 1,000,000 normal. This is by far the most sensitive test yet developed for MRD in HCL. We are prospectively cloning surface IgG in patients treated with BL22 to determine if BL22 can eradicate malignant cells after repeated cycles. We have also accumulated a large database of sequences of surface IgG expressed by HCL cells. We find significant differences in expressed IgG between patients with HCL and the poor prognosis variant, HCLv, with respect to both gene usage and mutation frequency. LMB-2 is an anti-CD25 recombinant immunotoxin which we previously showed in phase I testing was active in HCL, CLL, cutaneous T-cell lymphoma (CTCL), adult T-cell leukemia (ATL), and Hodgkin's disease (HD). We are currently testing LMB-2 in phase II trials in patients with CLL, CTCL and HCL. We have observed clinical responses in these trials. To determine if chemotherapy can prevent immunogenicity of immunotoxins, a trial is opening soon in which patients with adult T-cell leukemia will be treated with fludarabine and cyclophosphamide, followed by LMB-2. We have completed a phase I trial of the anti-mesothelin recombinant immunotoxin SS1P in patients with mesotheliomas, ovarian carcinoma, and pancreatic carcinoma. SS1P was administered by continuous infusion. Targeting solid tumors was much more challenging than targeting hematologic tumors, since patients were more likely to develop neutralizing antibodies, and tumors were less likely to be penetrated by immunotoxin. Nevertheless, evidence of antitumor activity was observed in several patients. Lessons learned in these studies should be useful in improving the treatment of patients with both solid and hematologic tumors lacking effective alternative therapies.